Radially arranged metal contact fingers for solar cells

ABSTRACT

A solar cell includes negative metal contact fingers and positive metal contact fingers. The negative metal contact fingers are interdigitated with the positive metal contact fingers. The metal contact fingers, both positive and negative, have a radial design where they radially extend to surround at least 25% of a perimeter of a corresponding contact pad. The metal contact fingers have bend points, which collectively form a radial pattern with a center point within the contact pad. Exactly two metal contact pads merge into a single leading metal contact pad that is wider than either of the exactly two metal contact pads.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally tosolar cells. More particularly, embodiments of the subject matter relateto solar cell metal contact fingers.

BACKGROUND

Solar cells are well known devices for converting solar radiation toelectrical energy. A solar cell includes P-type and N-type diffusionregions. Solar radiation impinging on the solar cell creates electronsand holes that migrate to the diffusion regions, thereby creatingvoltage differentials between the diffusion regions. Metal contactfingers are electrically coupled to the diffusion regions. An externalelectrical circuit, in turn, may include leads that are coupled to themetal contact fingers to allow the electrical circuit to be powered bythe solar cell. The present invention provides metal contact fingerarrangements that help improve solar cell efficiency.

BRIEF SUMMARY

In one embodiment, a solar cell includes negative metal contact fingersand positive metal contact fingers. The negative metal contact fingersare interdigitated with the positive metal contact fingers. The metalcontact fingers, both positive and negative, have a radial design wherethey radially extend to and surround at least 25% of a perimeter of acorresponding contact pad. The metal contact fingers have bend points,which collectively form a radial pattern with a center point within thecontact pad. Exactly two metal contact pads merge into a single leadingmetal contact pad that is wider than either of the exactly two metalcontact pads.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following drawings, wherein like reference numbersrefer to similar elements throughout the drawings.

FIG. 1 shows a plan view of a backside of a solar cell in accordancewith an embodiment of the present invention.

FIG. 2 shows the solar cell of FIG. 1 with solar cell interconnects inaccordance with an embodiment of the present invention.

FIGS. 3 and 4 show magnified views of a corner negative contact pad ofthe solar cell of FIG. 1.

FIGS. 5 and 6 show magnified views of a center negative contact pad ofthe solar cell of FIG. 1.

FIGS. 7 and 8 show magnified views of a center positive contact pad ofthe solar cell of FIG. 1.

FIGS. 9 and 10 show magnified views of a corner positive contact pad ofthe solar cell of FIG. 1.

FIG. 11 shows a cross-section view of the solar cell of FIG. 1 inaccordance with an embodiment of the present invention.

FIG. 12 shows a flow diagram of a method of arranging metal contactfingers of a solar cell in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of structures, materials, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details. In other instances,well-known details are not shown or described to avoid obscuring aspectsof the invention.

FIG. 1 shows a plan view of a backside of a solar cell 100 in accordancewith an embodiment of the present invention. In the example of FIG. 1,the solar cell 100 is a backside junction solar cell in that both itsdiffusion regions and the metal contact fingers coupled to the diffusionregions are on the backside of the solar cell 100. The backside of thesolar cell 100 is opposite the front side that faces the sun duringnormal operation.

The solar cell 100 includes a plurality of negative contact pads 110(i.e., 110-1, 110-2, and 110-3) and positive contact pads 120 (i.e.,120-1, 120-2, and 120-3). A contact pad provides a surface on which anexternal interconnect lead may be attached, e.g., by soldering, toconnect the solar cell 100 to another solar cell or an externalelectrical circuit, such as a load. The solar cell 100 has a negativeedge 112 and a positive edge 122. As its name implies, the negative edge112 is the edge of the solar cell 100 where the negative contact pads110 are located. The negative contact pads 110-1 and 110-3 are cornercontact pads, and the negative contact pad 110-2 is a center contactpad. Similarly, the positive edge 122 is the edge of the solar cell 100where the positive contact pads 110 are located. The positive contactpads 120-1 and 120-3 are corner contact pads, and the positive contactpad 120-2 is a center contact pad. In general, a solar cell may havemore or fewer contact pads. Within the same solar cell 100, the negativecontact pads 110 electrically connect to negative metal contact fingersbut not to positive metal contact fingers, and the positive contact pads120 electrically connect to positive metal contact fingers but not tonegative metal contact fingers.

To form serially connected solar cells as in a solar cell module, thenegative contact pads 110 of the solar cell 100 may be coupled topositive contact pads of another solar cell, and so on. FIG. 2 shows thesolar cell 100 with solar cell interconnects 220 in accordance with anembodiment of the present invention. The positive edge 122 and thenegative edge 112 are labeled in FIG. 2 to assist in locating thenegative contact pads 110 and the positive contact pads 120, which arenot labeled in FIG. 2 to avoid cluttering the drawing. An interconnect220 couples the solar cell 100 to another solar cell. In the example ofFIG. 2, an interconnect 220 has tabs 221 that are attached tocorresponding contact pads. For example, a tab 221 may be soldered ontoa positive contact pad 120 of the solar cell 100, and an opposing tab221 of the same interconnect 220 may be soldered onto a negative contactpad of another solar cell (not shown). The same applies for theinterconnect 220 attached to negative contact pads 110 on the negativeedge 112. Other example interconnects that may be employed include thosedisclosed in commonly-assigned U.S. Pat. No. 8,148,627, which isincorporated herein by reference in its entirety.

FIGS. 3 and 4 show magnified views of the corner negative contact pad110-1. FIG. 3 shows the negative contact pad 110-1 without labels toprovide an uncluttered drawing for reference. FIG. 4 shows the same viewas FIG. 3 but with labels for pointing out features of the solar cell100. Still, not all features are labeled in FIG. 4 for clarity ofillustration. In the example of FIGS. 1-10, a white space within thesolar cell 100 is covered by a metallic material (e.g., copper) and ablack space between white spaces represents an electrical insulator.

The solar cell 100 comprises negative metal contact fingers 401 (i.e.,401-1, 401-2, 401-3, etc.) that are coupled to corresponding negativecontact pads 110. In the example of FIG. 4, the perimeter of thenegative contact pad 110-1 has been bounded by dashes for illustrationpurposes. In one embodiment, to minimize resistive loses, exactly twonegative metal contact fingers 401 are merged into a single leadingnegative metal contact finger 403 (i.e., 403-1, 403-2, 403-3, etc.) thatis wider than either of the exactly two negative metal contact fingers401. The leading negative metal contact finger 403 is, in turn, coupledto a corresponding negative contact pad 110. As an example, negativemetal contact fingers 401-1 and 401-2 merge into a leading negativemetal contact finger 403-1, which in turn is coupled to the negativecontact pad 110-1. The leading negative metal contact finger 403-1 iswider than either the negative metal contact finger 401-1 or 401-2.Other examples that are labeled in FIG. 4 include negative metal contactfingers 401-3 and 401-4 merging together to form the leading negativemetal contact finger 403-2 and negative metal contact fingers 401-5 and401-6 merging together to form the leading negative metal contact finger403-3. The leading negative metal contact fingers 403-2 and 403-3 bothextend to the negative contact pad 110-1. The leading negative metalcontact finger 403-2 is wider than either the negative metal contactfinger 401-3 or 401-4. Similarly, the leading negative metal contactfinger 403-3 is wider than either the negative metal contact finger401-5 or 401-6.

The negative metal contact fingers 401 and 403 are so named because theyare coupled to corresponding N-type diffusion regions. The solar cell100 further comprises positive metal contact fingers 451 (i.e., 451-1,451-2, 451-3, etc.) and 453 (i.e., 453-1, 453-2, 453-3, etc.; see FIG.8) that are coupled to corresponding P-type diffusion regions. In oneembodiment, the solar cell 100 comprises interdigitated metal contactfingers. In particular, the negative metal contact fingers 401 areinterdigitated with positive metal contact fingers 451. This is shown inFIG. 4 with the positive metal contact finger 451-1 being between thenegative metal contact fingers 401-1 and 401-2, the positive metalcontact finger 451-2 being between the negative metal contact fingers401-3 and 401-4, and the positive metal contact finger 451-3 beingbetween the negative metal contact fingers 401-5 and 401-6. It is to benoted that for improved efficiency, in the case of an N-type siliconsubstrate, the positive metal contact fingers 451 and correspondingP-type diffusion regions (i.e., emitter diffusion region) are preferablymade as wide as possible between negative metal contact fingers 401.

In one embodiment, the negative metal contact fingers 401 are straightand parallel along the middle portion of the solar cell 100 but are bentto radially approach or extend toward a corresponding negative contactpad 110. This is illustrated in FIG. 4 with the bend points 402 (402-1,402-2, 402-3, etc.) of the negative metal contact fingers 401collectively forming a radial pattern with a center point that is withinthe negative contact pad 110. The bends 402 allow the metal contactfingers 401 to radially approach or extend to the negative contact pad110. In the example of FIG. 4, the radial pattern of the bend points 402is illustrated by dashed lines 421 and 422. In one embodiment, theradial pattern has a circumference that covers at least 25% or between25% and 75% of the perimeter of the negative contact pad 110. The radialdesign together with the merging of two metal contact fingers into onemetal contact finger helps increase efficiency by maximizing electricalcurrent collection around the contact pad and decreasing dead spacewhere little or no electrical current can be extracted.

The just described features of the negative metal contact fingers 401,leading negative metal contact fingers 403, and negative contact pads110 are generally present in the solar cell 100, including in positivemetal contact fingers 451, leading positive metal contact fingers 453,and positive contact pads 120.

FIGS. 5 and 6 show magnified views of the center negative contact pad110-2. FIG. 5 shows the negative contact pad 110-2 without labels. FIG.6 shows the same view as FIG. 5 but with labels for pointing outfeatures of the solar cell 100. Not all features are labeled in FIG. 6for clarity of illustration.

With reference to FIG. 6, negative metal contact fingers 401-7 and 401-8merge together to form a leading negative metal contact finger 403-4,which is wider than either the negative metal contact finger 401-7 orthe negative metal contact finger 401-8. The negative metal contactfingers 401-7 and 401-8 bend at bend points 402-7 and 402-8,respectively, such that both of the negative metal contact fingers 401-7and 401-8 and the leading negative metal contact finger 403-4 extend andpoint toward the negative contact pad 110-2 in a radial manner. Thenegative metal contact fingers 401-7 and 401-8 are interdigitated withthe positive metal contact finger 451-4, which is between the negativemetal contact fingers 401-7 and 401-8.

As shown in FIG. 6, the negative metal contact fingers 401 have a radialdesign where their respective bend points 402 (e.g., see bend points402-7, 402-8, 402-9, and 402-10) collectively form a radial pattern witha center point within a negative contact pad 110, which in the exampleof FIG. 6 is the negative contact pad 110-2. The radial pattern isillustrated by dashed lines 423, 424, and 425. In the example of FIG. 6,the radial pattern has a circumference that covers 75% of the perimeterof the negative contact pad 110-2, which is generally bounded by dashes.That is, in the example of FIG. 6, the negative metal contact fingers401 and 403 point to and surround 75% of the perimeter of the negativecontact pad 110-2. The increased radial coverage compared to that inFIG. 4 is due to the central location of the negative contact pad 110-2.

FIGS. 7 and 8 show magnified views of the center positive contact pad120-2. FIG. 7 shows the positive contact pad 120-2 without labels. FIG.8 shows the same view as FIG. 7 but with labels to point out features ofthe solar cell 100. Not all features are labeled in FIG. 8 for clarityof illustration.

With reference to FIG. 8, positive metal contact fingers 451-5 and 451-6merge together to form a leading positive metal contact finger 453-1. Todecrease resistive loses, the leading positive metal contact finger453-1 is wider than either the positive metal contact finger 451-5 orthe positive metal contact finger 451-6. For increased electricalcurrent collection, the positive metal contact fingers 451-5 and 451-6bend at bend points 402-11 and 402-12, respectively, such that both ofthe positive metal contact fingers 451-5 and 451-6 and the leadingpositive metal contact finger 453-1 radially extend and point toward thepositive contact pad 120-2. The positive metal contact fingers 451-5 and451-6 are interdigitated with the negative metal contact finger 401-9,which is between the positive metal contact fingers 451-5 and 451-6.Similarly, the positive metal contact fingers 451-7 and 451-8 merge toform the leading positive metal contact finger 453-2, with a negativemetal contact finger 401-10 being between the positive metal contactfingers 451-7 and 451-8.

Like the negative metal contact fingers 401, the positive metal contactfingers 451 radially approach or extend to a positive contact pad 110.In the example FIG. 8, the positive metal contact fingers 451 have aradial design where their respective bend points 402 (e.g., see bendpoints 402-11, 402-12, 402-13, and 402-14) collectively form a radialpattern with a center point within a positive contact pad 120, which inthe example of FIG. 8 is the positive contact pad 120-2. The radialpattern is illustrated by dashed lines 426, 427, and 428. In the exampleof FIG. 8, the radial pattern has a circumference that covers 75% of theperimeter of the positive contact pad 120-2, which has been generallybounded by dashes. In other words, in the example of FIG. 8, thepositive metal contact fingers 451 and 453 point to and surround 75% ofthe perimeter of the positive contact pad 120-2.

FIGS. 9 and 10 show magnified views of the corner positive contact pad120-1. FIG. 9 shows the positive contact pad 120-1 without labels. FIG.10 shows the same view as FIG. 9 but with labels to point out featuresof the solar cell 100. Not all features are labeled in FIG. 10 forclarity of illustration.

With reference to FIG. 10, positive metal contact fingers 451-9 and451-10 merge together to form a leading positive metal contact finger453-3. The leading positive metal contact finger 453-3 is wider thaneither the positive metal contact finger 451-9 or the positive metalcontact finger 451-10. The positive metal contact fingers 451-9 and451-10 bend at bend points 402-17 and 402-18, respectively, such thatboth of the positive metal contact finger 451-9, positive metal contactfinger 451-10, and the leading positive metal contact finger 453-3radially extend and point toward the positive contact pad 120-1. Thepositive metal contact fingers 451-9 and 451-10 are interdigitated withthe negative metal contact finger 401-11, which is between the positivemetal contact fingers 451-9 and 451-10.

As shown in FIG. 10, the positive metal contact fingers 451 have aradial design where their respective bend points 402 (e.g., see bendpoints 402-15, 402-16, 402-17, and 402-18) collectively form a radialpattern with a center point within a positive contact pad 120, which inthe example of FIG. 10 is the positive contact pad 120-1. The radialpattern is illustrated by dashed lines 429 and 430. In the example ofFIG. 10, the radial pattern has a circumference that covers at least 25%of the perimeter of the positive contact pad 120-1, which has beengenerally bounded by dashes. In the example of FIG. 10, the positivemetal contact fingers 451 and 453 point to and surround 25% of theperimeter of the positive contact pad 120-1.

FIG. 11 shows a cross-section view of the solar cell 100 in accordancewith an embodiment of the present invention. The solar cell 100 is abackside junction solar cell in that its N-type diffusion regions 601and P-type diffusion regions 602 are on the backside of the solar cell.During normal operation, the front side of the solar cell 100 faces thesun to collect solar radiation. As shown in FIG. 11, the negative metalcontact fingers 401 electrically connect to the N-type diffusion regions601, and the positive metal contact fingers 451 electrically connect tothe P-type diffusion regions 602 on the backside (only one positivemetal contact finger and P-type diffusion region are shown for clarityof illustration). The spaces 610 may be filled with an electricalinsulator (e.g., a dielectric) to isolate the negative metal contactfingers 401 from the positive metal contact fingers 451. The N-typediffusion regions 601 and P-type diffusion regions 602 may be formed ina substrate 603, or in another layer (e.g., polysilicon) formed on thesubstrate 603. The metal contact fingers 401 and 451 may comprise asingle layer of metal (e.g., aluminum) or a stack of metals (e.g.,copper/barrier layer/aluminum).

In one embodiment, the substrate 603 comprises an N-type siliconsubstrate. Accordingly, in that embodiment, the N-type diffusion regions601 serve as the base of the solar cell that collects majority chargecarriers, and the P-type diffusion regions 602 serve as the emitter ofthe solar cell that collects minority charge carriers. In anotherembodiment where the substrate 603 comprises a P-type silicon substrate,the P-type diffusion regions 602 serve as the base of the solar cellthat collects majority charge carriers, and the N-type diffusion regions601 serve as the emitters of the solar cell that collect minority chargecarriers.

FIG. 12 shows a flow diagram of a method of arranging metal contactfingers of a solar cell in accordance with an embodiment of the presentinvention. As can be appreciated, the steps of the method of FIG. 12 maybe performed at the same time using appropriate masking and etchingtechniques, for example. In particular, a metal contact finger mask maybe designed such that metal contact fingers radially extend tocorresponding contact pads.

In the example of FIG. 12, the method includes interdigitating aplurality of negative metal contact fingers with a plurality of positivemetal contact fingers, the negative metal contact fingers beingelectrically connected to N-type diffusion regions on a backside of asolar cell, the positive metal contact fingers being electricallyconnected to P-type diffusion regions on the backside of the solar cell,the solar cell including a front side that faces the sun during normaloperation (step 701). The method further includes arranging the negativemetal contact fingers to have bend points that form a radial patternhaving a center point within a negative contact pad of the solar cell,the negative metal contact fingers radially extending to and surroundingat least 25% of a perimeter of the negative contact pad (step 702). Themethod yet further includes arranging the positive metal contact fingersto have bend points that form a radial pattern having a center pointwithin a positive contact pad of the solar cell, the positive metalcontact fingers radially extending to and surrounding at least 25% of aperimeter of the positive contact pad (step 703).

Exactly two negative metal contact fingers may be arranged to merge intoa single leading negative metal contact finger that extends to thenegative contact pad. The single leading negative metal contact fingermay be formed to be wider than either of the exactly two negative metalcontact fingers. The negative metal contact fingers may be arranged topoint to and surround between 25% and 75% of the perimeter of thenegative contact pad. An external interconnect lead may be soldered ontothe negative contact pad. The solar cell may be serially connected toanother solar cell by a solar cell interconnect that electricallyconnects the negative contact pad to another negative contact pad of theother solar cell. The positive metal contact fingers and positivecontact pads of the solar cell may have the same features as theirnegative counterparts.

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure.

What is claimed is:
 1. A solar cell comprising: a plurality of positivemetal contact fingers, each of the positive metal contact fingers beingcoupled to one or more P-type diffusion regions on a backside of thesolar cell, a front side of the solar cell facing the sun during normaloperation to collect solar radiation; a plurality of negative metalcontact fingers, each of the negative metal contact fingers beingcoupled to one or more N-type diffusion regions on the backside of thesolar cell, the negative metal contact fingers being interdigitated withthe positive metal contact fingers; and a negative contact pad at acorner of the solar cell and providing a surface on which an externalinterconnect lead may be attached to electrically couple to an N-typediffusion region by way of the negative metal contact fingers, thenegative metal contact fingers having bend points that form a radialpattern having a center point within the negative contact pad, thenegative metal contact fingers with the bend points radially extendingto surround greater than 25% of a perimeter of the negative contact pad.2. The solar cell of claim 1 further comprising a positive contact padproviding a surface on which an external interconnect lead may beattached to electrically coupled to a P-type diffusion region by way ofthe positive metal contact fingers, the positive metal contact fingershaving bend points that form a radial pattern having a center pointwithin the positive contact pad, the positive metal contact fingersradially extending to surround at least 25% of a perimeter of thepositive contact pad.
 3. The solar cell of claim 1 wherein exactly twonegative metal contact fingers merge into a single leading negativemetal contact finger that extends to the negative contact pad.
 4. Thesolar cell of claim 3 wherein the single leading negative metal contactfinger is wider than either of the exactly two negative metal contactfingers.
 5. The solar cell of claim 1 wherein the negative metal contactfingers radially extend to surround between 25% and 75% of the perimeterof the negative contact pad.
 6. The solar cell of claim 1 wherein theexternal interconnect lead is soldered onto the negative contact pad. 7.The solar cell of claim 1 wherein the solar cell is serially connectedto another solar cell by a solar cell interconnect that electricallyconnects the negative contact pad to a positive contact pad of the othersolar cell.
 8. A solar cell comprising: a plurality of negative metalcontact fingers that are interdigitated with a plurality of positivemetal contact fingers, each of the positive metal contact fingers beingcoupled to a corresponding P-type diffusion region of the solar cell,each of the negative metal contact fingers being coupled to acorresponding N-type diffusion region of the solar cell; and a negativecontact pad at a corner of the solar cell and electrically connected tothe negative metal contact fingers but not to the positive metal contactfingers, the negative metal contact fingers being arranged with bendpoints to radially extend to surround greater than 25% of a perimeter ofthe negative contact pad.
 9. The solar cell of claim 8 wherein thenegative metal contact fingers with bend points radially extend tosurround between 25% and 75% of a perimeter of the negative contact pad.10. The solar cell of claim 8 further comprising a positive contact padelectrically connected to the positive metal contact fingers but not tothe negative metal contact fingers, the positive metal contact fingersbeing arranged to radially extend to the positive contact pad.
 11. Thesolar cell of claim 8 wherein exactly two negative metal contact fingersmerge into a single leading negative metal contact finger that radiallyextends to the negative contact pad.
 12. The solar cell of claim 8wherein the single leading negative metal contact finger is wider thaneither of the exactly two negative metal contact fingers.
 13. The solarcell of claim 8 wherein an external interconnect lead is soldered ontothe negative contact pad.
 14. The solar cell of claim 8 wherein thesolar cell is serially connected to another solar cell by a solar cellinterconnect that electrically connects the negative contact pad to apositive contact pad of the other solar cell.
 15. A method of arrangingmetal contact fingers of a solar cell, the method comprising:interdigitating a plurality of negative metal contact fingers with aplurality of positive metal contact fingers, the negative metal contactfingers being electrically connected to N-type diffusion regions on abackside of a solar cell, the positive metal contact fingers beingelectrically connected to P-type diffusion regions on the backside ofthe solar cell, the solar cell including a front side that faces the sunduring normal operation; arranging the negative metal contact fingers tohave bend points that form a radial pattern having a center point withina negative contact pad at a corner of the solar cell, the negative metalcontact fingers with the bend points radially extending to andsurrounding greater than 25% of a perimeter of the negative contact pad;and arranging exactly two negative metal contact fingers to merge into asingle leading negative metal contact finger that extends to thenegative contact pad.
 16. The method of claim 15 further comprising:arranging the positive metal contact fingers to have bend points thatform a radial pattern having a center point within a positive contactpad of the solar cell, the positive metal contact fingers radiallyextending to and surrounding at least 25% of a perimeter of the positivecontact pad.
 17. The method of claim 15, wherein the single leadingnegative metal contact finger is formed to be wider than either of theexactly two negative metal contact fingers.
 18. The method of claim 15further comprising: soldering an external interconnect lead onto thenegative contact pad.
 19. The method of claim 15 further comprisingserially-connecting the solar cell to another solar cell by a solar cellinterconnect that electrically connects the negative contact pad to apositive contact pad of the other solar cell.